1. Field of the Invention
The field of the invention relates to a high-voltage, high-power ultracapacitor energy storage pack composed of a large number of serially connected individual low-voltage ultracapacitor cells that store an electrical charge.
2. Background of the Invention
The connecting together of individual battery cells for high-voltage, high-energy applications is well known. However, the chemical reaction that occurs internal to a battery during charging and discharging typically limits deep-cycle battery life to hundreds of charge/discharge cycles. This characteristic means that the battery pack has to be replaced at a high cost one or more times during the life of a hybrid-electric or all-electric vehicle.
Batteries are somewhat power-limited because the chemical reaction therein limits the rate at which batteries can accept energy during charging and supply energy during discharging. In a hybrid-electric vehicle application, the battery power limitation manifests itself as an internal series resistance that restricts the drive system efficiency in capturing braking energy through regeneration and supplying power for acceleration.
Ultracapacitors are attractive because they can be connected together, similar to batteries, for high-voltage applications; have an extended life of hundreds of thousands of charge/discharge cycles; and have a low equivalent internal series resistance that allows an ultracapacitor pack to accept and supply much higher power than similar battery packs. Although ultracapacitor packs may be more expensive than battery packs for the same applications and cannot store as much energy as battery packs, ultracapacitor packs are projected to last the life of the vehicle and offer better fuel-efficient operation through braking regeneration energy capture and supplying of vehicle acceleration power. Furthermore, the price of an ultracapacitor pack has the potential to decrease significantly because of economies of scale in known manufacturing techniques.
During charging and discharging operation of the ultracapacitors, parasitic effects, as modeled by the equivalent series resistance, cause the cell temperature to increase. Cooling is required to minimize increased temperature operation that would degrade the energy storage and useful life of each ultracapacitor.
Low-voltage energy cells, batteries, or ultracapacitors are connected in series to obtain high-voltage energy storage. Because of variations in materials and manufacturing, energy storage cells are not perfectly matched. As the serially connected pack operates through multiple charge and discharge cycles, the cell differences cause the energy storage to become more and more imbalanced among the cells. The energy storage imbalance from cell to cell limits the performance of the overall pack and can shorten the life of the individual cells.
Packs of batteries and packs of ultracapacitors have been built in various forms and configurations. Various different wiring harnesses, buss bars, and connections have been used for current routing and voltage monitoring. Various different types of circuits for charging, discharging, and equalizing have also been built. Energy storage cells have been mounted in various “egg crate” or “wine rack” style vertical and horizontal support structures. High-voltage packages contain batteries enclosed within a single pack. Batteries have even been connected together by simply touching under some pressure the positive end of one battery against the negative end of another battery such as can be found in flashlights, small toys and appliances. High-energy packs usually include some form of convection air or liquid cooling.